1. Technical Field of the Invention
This invention relates generally to wireless communications and more particularly to circuits used to support wireless communications.
2. Description of Related Art
Communication systems are known to support wireless and wireline communications between wireless and/or wireline communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks to radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, 3GPP, LTE, LTE Advanced, RFID, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID reader, RFID tag, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to one or more antennas (e.g., MIMO) and may include one or more low noise amplifiers, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier(s) receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
Currently, wireless communications occur within licensed or unlicensed frequency spectrums. For example, wireless local area network (WLAN) communications occur within the unlicensed Industrial, Scientific, and Medical (ISM) frequency spectrum of 900 MHz, 2.4 GHz, and 5 GHz. While the ISM frequency spectrum is unlicensed there are restrictions on power, modulation techniques, and antenna gain. Another unlicensed frequency spectrum is the V-band of 55-64 GHz.
In wireless transmitters, digital-to-analog converters (DACs) are implemented in various ways in an attempt to provide low-noise (e.g., thermal noise) analog output signals for amplification and transmission through one or more antennas. Traditionally, this has been achieved by using, for example, a switched-capacitor DAC.
One example of this structure is provided by Vital et al., “Integrated Mixed-Mode Digital-Analog Filter Converters”, IEEE J. Solid-State Circuits June 1990 p 660+. However, while providing potentially low thermal noise, the settling can be too slow for GHz clock frequencies. A second arrangement includes non-return-to-zero current DAC cells with the outputs in parallel. In this circuit, summing of outputs follows from Kirchoffs' current law. An example of this structure is provided by Su & Wooley, “A CMOS Oversampling D/A Converter with a Current-Mode Semidigital Reconstruction Filter”, IEEE J. Solid-State Circuits December 1993 p. 1224+. This example, however, includes at least two limitations such as nonlinear “intersymbol interference” between outputs of a DAC on successive cycles as well as high thermal noise.
Some early examples of DACs used in delta-sigma or sigma-delta modulators, include: Okamoto et al. “A Stable High-Order Delta-Sigma Modulator with an FIR Spectrum Distributor”, IEEE J. Solid-State Circuits July 1993 p. 730+ and Harrison & Weste “A multi-bit sigma-delta ADC with an FIR DAC loop filter”, IEEJ Intl Analog Workshop, 2000.
An early example of DACs used in bandpass RF DACs for transmitters including a combination of a DAC with a digital EA modulator, digital subtractor and error DAC is shown in Barkin et al., “A CMOS Oversampling Bandpass Cascaded D/A Converter With Digital FIR and Current-Mode Semi-Digital Filtering”, IEEE J Solid-State Circuits April 2004 p. 585+.
An example of a “two-element” DAC is shown in Harrison et al., “An LC Bandpass ΔΣ ADC with 70 dB SNDR Over 20 MHz Bandwidth Using CMOS DACs”, ISSCC 2012 paper 8.1 as well as in Gao et al., “Excess Loop Delay Effects in Continuous-Time Delta-Sigma Modulators and the Compensation Solution”, IEEE Intl Symposium on Circuits & Systems, 1997 p 65+.
Disadvantages of conventional approaches will be evident to one skilled in the art when presented in the disclosure that follows.